A lithium ion secondary battery is characterized by being small in size and large in capacity and widely used as a power supply for cellular phones, notebook computers and others. However, recently in the circumstances where mobile electronics have been rapidly advanced and use of lithium ion secondary batteries in electric cars has been realized, a further improvement of energy density is desired. As a positive electrode active material of a lithium ion secondary battery, LiCoO2 and LiNiO2 are well known. However, raw materials for these positive electrode active materials are expensive and, in addition, a safety in a charge state is questioned. These problems are made apparent particularly when the battery is used in large-size products such as automobiles.
As another positive electrode active material, a lithium manganese composite oxide, LiMn2O4 having a spinel-type crystal structure has been aggressively studied. LiMn2O4 is regarded as a promising positive electrode active material for a lithium secondary battery since a raw material Mn is abundantly present as a resource and provided at relatively low cost. Besides these, LiMn2O4 is highly stable during overcharge and at a high temperature. However, LiMn2O4 deteriorates with cycles and causes capacity drop at a high temperature. This is conceivably caused by instability of Mn3+. To describe more specifically, when the average valence of Mn ion changes between a trivalent state and a quadrivalent state, Jahn-Teller strain generates in a crystal and lowers stability of a crystal structure. Due to this, performance and the like presumably deteriorate with cycles.
In order to reduce the Jahn-Teller strain, study has been made for substituting Mn with another element. Substitution of Mn with another element reinforces binding force. Patent Literature 1 discloses a positive electrode active material represented by LiMn2O4 in which Mn3+ is substituted with another metal. To describe more specifically, Patent Literature 1 describes a secondary battery having a manganese composite oxide having a spinel structure and represented by a composition formula, LiMxMn2−xO4 (M is one or more selected from Al, B, Cr, Co, Ni, Ti, Fe, Mg, Ba, Zn, Ge and Nb; and 0.01≦x≦1). Furthermore, a case where LiMn1.75Al0.25O4 is used as a positive electrode active material is specifically disclosed.
Furthermore, a lithium manganese composite oxide has a discharge potential of 4.2 V or less and a low discharge capacity. Thus, increasing energy density is a technical problem. As a method for improving the energy density of a lithium ion secondary battery, a method for increasing the operating potential of the battery is effective. It has been so far known that a 5 V-level operating potential can be realized by substituting a part of Mn of LiMn2O4 with an element such as Ni, Co, Fe, Cu and Cr (for example, Patent Literature 2, Non Patent Literature 1 and Non Patent Literature 2). Of the substitution elements that can realize such a 5 V-level operating, particularly an element using Fe is favorable in view of resource, environment and cost, and demand is expected to increase in various industrial fields including the automobile industry.
By substituting Mn with Fe, Mn is present in a quadrivalent state, discharge is caused by the reaction of Fe3+→Fe4+ in place of the oxidation-reduction reaction of Mn3+→Mn4+. Since the reaction of Fe3+→Fe4+ has a high potential of 4.5 V or more, it can be expected to function as a 5 V-level electrode material. In Patent Literature 2, a manganese iron lithium composite oxide of a spinel-type structure, which is represented by Li[Fe1/2+xMeyMn3/2−x−y]O4 (note that, 0≦x, 0<y, x+y≦½; and Me is represented by one or two or more of Cr, Co and Al) is synthesized to realize a positive electrode active material having an oxidation-reduction potential of about 5 V.